The present invention relates to a composition for thermal energy storage, and more particularly, to a composition comprising a phase change material and silica particles, where the composition is in the form of a reversible gel which exhibits high thermal conductivity and high thermal storage capacity, and which becomes a hard solid at temperatures below the crystalline freezing temperature of the phase change component.
Phase change materials are known which may be repeatedly converted between solid and liquid phases and utilize their latent heats of fusion to absorb, store and release energy to heat or cool during such phase conversions. These latent heats of fusion are greater than the sensible heat capacities of the materials. For example, in phase change materials, the amount of energy absorbed upon melting or released upon freezing is much greater than the amount of energy absorbed or released upon increasing or decreasing the temperature of the material over an increment of 10.degree. C.
Upon melting and freezing, per unit weight, a phase change material (PCM) absorbs or releases substantially more energy than a sensible heat storage material that is heated or cooled over generally the same temperature range. In contrast to a sensible heat storage material that absorbs and releases energy essentially uniformly over a broad temperature range, a phase change material absorbs and releases a large quantity of energy in the vicinity of its melting/freezing point.
Phase change materials capable of storing and releasing thermal energy have found many applications including beverage and food containers, medical wraps, and textile applications such as garments. However, one of the basic problems in the use of solid-to-liquid phase change materials for control of temperature is containment. That is, when the phase change material is in the liquid phase, it may leak out of any small holes in the vessel or pouch in which it is contained. Further, problems are encountered from expansion and contraction as the phase change material changes from a solid to liquid state, and from a liquid to solid state.
My U.S. Pat. No. 5,106,520 discloses a thermal energy storage material in the form of a dry powder formed from an alkyl hydrocarbon or water phase change material and silica particles. That composite eliminates this problem in that it does not liquify when the phase change material is heated above its melting point and will not form a rigid solid at temperatures below the melting point. Such a PCM/silica mix is useful as a hot or cold medical wrap.
However, the manufacture of such PCM/silica dry powders requires intensive mixing steps. Further, there are applications where it is desirable to obtain a higher rate of heat transfer than can be provided with the use of PCM/silica dry powders. For example, a higher rate of heat transfer would be beneficial in applications where it is desirable to maintain low temperatures, such as in shipping or refrigeration. In addition, because the PCM/silica dry powders do not form a rigid solid, it is difficult to determine when the composition has reached a frozen state.
Accordingly, there is still a need in the art for a thermal energy storage composition which has a high thermal storage capacity, which may be produced economically, and which may be used in a variety of applications. There is a further need in the art for a composition which will not leak out of the vessel in which it is contained, and which will form a rigid solid when cooled below the crystallization temperature, thereby providing a clear indication of conversion to the frozen state.